An example of an application for such a sensor is constituted by a generator or network circuit breaker mounted in a case of metal cladding, or a substation in a metal case, the case containing sulfur hexafluoride SF.sub.6 at a pressure of a few bars. The density sensor is fixed to the case from the outside and serves to monitor the rate at which the dielectric gas leaks out from the case by comparing density readings made throughout the time the circuit breaker is in use. Since leaks are inevitable, even if very small, after several years of use, density tends towards a threshold value below which the operation of the circuit breaker or the switchgear is no longer reliable. It is then necessary to inject dielectric gas so as to raise the density to a nominal value, e.g. equal to 3.5 bars. When the threshold is crossed, it is the general practice to raise an alarm to cause action to be taken on the circuit breaker, specifically to proceed with injection of dielectric gas.
A density sensor comprises a pressure detector and a temperature detector disposed inside the fixing support so as to be in communication with the dielectric gas, and a measurement unit for calculating the density of the gas for each pair of pressure and temperature values P and T that are acquired at the same time.
Curve 21 in FIG. 1 relates to an experiment performed using a sensor of the type described above. The metal cladding case was installed on an operating site in the open air, which is the case of a large fraction of sites on which such an electrical switchgear is operated. The case extended in a longitudinal direction and in the experiment said direction was oriented east-west on the operating site. The density sensor was fixed on one end of the case so as to be exposed to solar radiation only in the afternoon. Curve 21 shows density as calculated for each pair of pressure and temperature readings obtained simultaneously, and it reveals two distinct kinds of behavior of the sensor. A first kind of behavior is characterized by the density remaining flat 21A at around the nominal value of 3.5 bars and corresponds to pairs of pressure and temperature readings made during the day and in the absence of significant solar radiation. A second kind of behavior which corresponds to readings performed in daytime and in the presence of significant solar radiation is characterized by daily variation 21B of the density, during which the density is initially greater than the nominal value and subsequently less than the nominal value, with the transition between the positive and negative parts of the variation corresponding substantially to the sun being at its zenith.
The real density of SF.sub.6 inside the case remained constant and equal to its nominal value, as is shown by the flat curve produced on each day that readings were taken in the absence of significant solar radiation. In fact, the daily variation of density in the presence of significant sunshine represents an artifact of measurement. Such an artifact does not prevent the rate of leakage from the case being monitored insofar as it is easy to make use only of readings performed in the absence of significant solar radiation when calculating density. However, a problem arises when the amplitude of the daily variation in the calculated density value on a day of significant sunshine drops significantly below the density threshold, as referenced at 20 in FIG. 1. This happens in particular when the density of the gas contained inside the case has in any event moved closer to the threshold after several years of operation because of the inevitable minimal leakage. When the threshold is crossed, an alarm is generated by the negative portion of the variation in density as calculated by the density sensor on a day of significant sunshine, and that alarm is considered to be untimely insofar as the density threshold will not genuinely be crossed for several more weeks or even several more months.